In existing lip-type seals mounted on a fixed housing and slidingly sealing the surface of a rotating shaft, attempts have been made, out of consideration for environmental concerns, to reduce friction via measures such as optimizing the shape, dimensions, and material of the lip; coating the lip sealing face or the counterpart shaft surface; or optimizing the roughness of the counterpart shaft surface.
In cases in which a coating is applied to the lip sealing face or the counterpart shaft surface or in which the roughness of the counterpart shaft surface is optimized, there is the problem that effects are only obtained initially, and low friction cannot be maintained due to wear over time.
Meanwhile, as an example of a shaft seal device with improved lubrication properties, the sealing means shown in FIG. 12 (hereafter, “first prior art”; see, for example, patent document 1) is known.
The first prior art incorporates a lip-type seal 51 attached to a housing 50, the lip-type seal 51 having a sealing edge 53 disposed so as to contact a rotating shaft 52, and is provided with an arrow-shaped groove 55 in which grooves 56 and ridges 57 are alternatingly provided within a contact zone 54 in which the sealing edge 53 contacts the surface of the rotating shaft. When the rotating shaft 52 rotates within the housing 50, the arrow-shaped groove 55 creates a pumping effect, repelling foreign matter intruding from the atmosphere side and maintaining a seal effect by pushing back fluid from a sealed fluid side.
Shaft seal devices provided with a spiral-shaped groove in lieu of the arrow-shaped groove of the first prior art are also known (for example, see patent document 2).
The sealing means shown in FIG. 13 (hereafter, “second prior art”; for example, see patent document 3) is further known as an example of a shaft seal device that balances torque reduction with seal properties.
The second prior art has a seal lip 60 for sealing off a sealed fluid and a screw pump mechanism 64 disposed on the atmosphere side of the seal lip 60 and comprising a screw 62 formed on the surface of a rotating shaft 61 and a cylindrical part 63, and the screw pump mechanism 64 exerts a fluid pumping action upon the seal lip 60 to substantially reduce the tension of the seal lip 60, thereby ensuring seal properties through the agency of the seal lip 60 and reducing torque on the seal lip 60 through the agency of the screw pump mechanism 64.
However, because the shaft seal device according to the first prior art shown in FIG. 12 is structured so that the sealing edge 53 of the lip-type seal 51 contacts the arrow-shaped groove 55 formed from a material of high hardness in the contact zone 54 on the surface of the rotating shaft, the sealing edge 53 wears down quickly. Moreover, the grooves 56 of the arrow-shaped groove 55 have a “V-shape” directly connecting the atmosphere side and the sealed fluid side along the axial direction of the shaft, and the apical part of the sealing edge 53 does not contact with the V-shaped grooves 56, so that the sealed fluid side and the atmosphere are in a state of constant communication, resulting in the possibility of sealed fluid leaking to the atmosphere side when the device is at rest. It is known that a shaft surface roughness of 2.5 μm or greater can cause leakage when at rest in normal oil seals. In the case of the first prior art, severe leakage will occur when the depth of the grooves 56 is 2.5 μm or greater.
A shaft seal device provided with a spiral-shaped groove in lieu of the arrow-shaped groove of the first prior art has problems similar to those of the first prior art, in that the shape of the spiral-shaped groove allows for direct communication between the atmosphere side and the sealed fluid side via the spiral-shaped groove.
Furthermore, in the shaft seal device according to the second prior art shown in FIG. 13, the reduction on the tension on the seal lip 60 in order to reduce the torque of the seal lip 60 leads to the problems of reductions in the seal properties of the seal lip 60 and infiltration of the atmosphere into the sealed fluid side.